When heated to the temperatures encountered in ordinary combustion, pyrolysis, and/or gasification processes, coal such as a typical eastern bituminous coal may exhibit one or more of the unfavorable reactions that include swelling, caking, agglomerating, and emission of sulfurous vapors.
Many proposals have been made for processes to remove sulfur from solid and liquid carbonaceous fuels before the fuels are burned or converted to other fuels or other products. Some of these proposals include the use of alkali metal sulfides. In the case of liquid materials, for example, U.S. Pat. No. 1,413,005, Cobb, discloses removing sulfur from petroleum oil with an alkaline earth sulfide compound prepared by mixing 800 pounds of freshly burned lime (CaO) with a mixture of 600 pounds of commercial sulfide of soda (Na.sub.2 S) in 417 pounds of water. The oil and the sulfide compound are agitated together, with steam, at 212.degree. to 300.degree. F. U.S. Pat. No. 2,020,661, Schulze, discloses removing sulfur from petroleum and hydrocarbon liquids with an aqueous solution of sodium monosulfide or polysulfides, using excess alkalinity provided by adding sodium hydroxide solution, at 40.degree. to 90.degree. F. Proposals of this kind deal with elemental sulfur and sulfur compounds already in the liquid form, such as liquid mercaptans.
Removal of sulfur from solid fuels is generally more difficult. Typically, U.S. Pat. No. 3,472,624, Ridley, discloses that coke can be desulfurized by reacting it with Na.sub.2 S, initially containing 4 percent water and hydrogen at 1,000.degree. and 1,400.degree. F. in a kiln, then washing with water. According to the patentee, a major advantage is that the Na.sub.2 S can be recycled directly, without expensive conversion to Na.sub.2 C0.sub.3 or NaOH. Such desulfurization reactions are carried out at temperatures higher than the critical temperature of water (ca 706.degree. F.) so that they take place in a dry state. The temperatures employed are considerably above the thermal degradation temperatures for bituminous coals, for example, so valuable volatile constituents are lost rather than retained in the product feedstocks. At such high temperatures, moreover, some of the remaining constituents may be transformed into even less reactive compounds.
In contrast, the hydrothermal processes employ a distinctive form of treatment, as outlined above, that significantly upgrades the quality of raw particulate coal. The improved process of U.S. Pat. No. 4,092,125, Stambaugh and Chauhan, supra, is particularly effective for desulfurization of the coal and/or for increasing the chemical reactivity and improving the physical behavioral characteristics of the product feedstock.
Although the feedstocks produced by these last-mentioned processes are of outstanding quality, for some uses they may be prohibitively expensive; in particular where the required feedstock characteristics are such that the processes must include regeneration or frequent replacement of the aqueous treating solution.
The present invention provides a considerably less expensive process, yet retains most of the advantages of the improved hydrothermal processes, supra. The free-swelling index of the coal can be reduced substantially to unity, and the resulting feedstock can be rendered substantially non-caking and non-agglomerating under the conditions of its use in an ordinary gasifier, combustor, or liquefaction plant. It is understood that such terms as free-swelling index (FSI), non-agglomerating (NA) and the like are used as defined in ASTM Test No. D-720-67. FSI is a measure of the caking and agglomerating characteristics of the feedstock, since coals with high FSI values invariably cake and agglomerate, whereas FSI values near unity indicate that the feedstock will pass freely through a coal utilization process such as that employed in a gasifier. The improved characteristics of the feedstock produced by a process according to this invention, together with the highly pervasive encatalyzation of the feedstock with the calcium, magnesium, or dolomite produce a highly reactive feedstock. The catalyst incorporated in the coal acts as a sulfur absorber during gasification or combustion.
While the process of the present invention is not designed to remove sulfur from the coal during the process of converting it to a feedstock, the pervasive presence in the feedstock of a considerable quantity of calcium and/or magnesium causes an increased quantity of sulfur to be chemically bound in the ash from the gasifier or combustor. This may avoid or reduce the requirements for gas scrubbers which could otherwise be mandatory where a raw coal contains substantial amounts of sulfur.